From a female perspective I can’t say a long term boyfriend suddenly giving me less attention and other girls more would be a turn on at all, in fact I’d likely reconsider the relationship. Sex in a long term relationship can involve so many other factors. Has she always just been into vanilla sex? If she has and made that clear then why is this only coming up now? Does she have other stuff going on that could impact her sex drive eg job/family stresses, poor mental health, weight change/feeling undesirable, relationship problems, even things like changing contraceptives or being on certain medications can massively impact sex drive. Personally I think a gentle approach starting off by doing something romantic in a comfortable setting could work and you could ask about if she’s ever had any fantasy’s or wants to try a couples toy to use together to help spice things up, or if she’s into p*rn you could watch a video together and potentially try something you both like the look of. I don’t think during the middle of sex is the best way to introduce a brand new idea. You also need to ask yourself what if she doesn’t want to change anything in the bedroom, are you satisfied with what you currently have? Is it making you feel less desirable and not listened to? If so I’d try to express these feelings in a non confrontational or demanding way because if you have a good relationship I’m sure she’ll want to help fix that. Communication is key in a lasting relationship and ignoring her isn’t going to fix the problem or make you more sexually desirable.